The present invention relates to an automatic needle thread supply control system for a sewing machine having a thread take-up member which takes up the needle thread in synchronism with the vertical reciprocatory motion of the needle and, more specifically, to an automatic needle thread supply control system having a pair of thread clamping members disposed between the thread supply source and the thread take-up member in the thread supply path extending from the thread supply source to the needle, for clamping and releasing the needle thread.
There have been proposed, to form satisfactory stitches, various sewing machines in which the thread supply is controlled so that points of interlock of the needle thread and the bobbin thread are located at the middle of the thickness of the fabric being sewn. Such a sewing machine disclosed, for example, in Japanese Patent Publication No. 58-10115 comprises a needle thread supply controller capable of temporarily releasing the needle thread to remove tension from the needle thread, and a detecting device for detecting the motion of the tension detector and actuating the needle thread supply controller, in which the needle thread supply controller releases the needle thread upon the arrival of the point of interlock of the needle thread and the bobbin thread at the middle of the thickness of the fabric being sewn. Another sewing machine disclosed in Japanese Patent Publication No. 60-19278 comprises an electromagnetic needle thread gripper disposed between the take-up lever and the needle, in which the needle thread gripper grips the needle thread to simultaneously stop pulling up the bobbin thread and to draw out the needle thread upon the arrival of the point of interlock of the needle thread and the bobbin thread at the middle of the thickness of the fabric being sewn.
In the known prior sewing machines described herein-before, however, the arrival of the point of interlock of the needle thread and the bobbin thread at the middle of the thickness of the fabric being sewn is not detected directly. Therefore, it is necessary, to locate the point of interlock of the needle thread and the bobbin thread accurately at the middle of the thickness of the fabric being sewn, that various factors which affect the needle thread tension, such as the type of fabric, the thickness of fabric, the type and thickness of thread, the width and length of stitch, and the type of pattern, are determined beforehand, the supply of the needle thread is calculated on the basis of those given factors, and the supply of the needle thread is controlled according to the result of the calculation. Accordingly, the sewing machine needs detectors for detecting those factors which affect the needle thread tension, and an arithmetic unit for calculating the supply of the needle thread on the basis of the results of the detection, and hence the sewing machine inevitably becomes complex and expensive.
A needle thread supply control system to obviate such inconveniences is disclosed, for example, in Japanese Patent Publication No. 53-41580. In this needle thread supply control system, a pair of tension discs are controlled by an actuator of the solenoid type for clamping and releasing the needle thread. The actuator is driven in synchronism with the main shaft of the sewing machine so as to permit the supply of the needle thread in a predetermined period in one stitching cycle and check the supply of the needle thread in the rest of the period. While the tension discs are released, the needle thread is supplied without restraint so that the supply of the needle thread is dependent only on the normal stitching conditions, such as the type of the fabric, stitch length and the type of pattern. After the needle thread has been supplied according to such normal stitching conditions, the tension discs are pressed together to check the supply of the needle thread in order to prevent the disarrangement of the stitch formed while the needle thread is supplied according to the normal stitching conditions. Thus, the needle thread supply control system eliminates the tension detector for detecting the tension of the needle thread and the device for calculating the supply of the needle thread, and hence simplifies the general constitution of the sewing machine.
However, this needle thread supply control system still has disadvantages. While the tension discs are released to supply the needle thread without restraint, the take-up lever swings from the uppermost position downward in synchronism with the vertical reciprocative motion of the needle. Consequently, part of the needle thread stored by the loop taker when the take-up lever is at the uppermost position is used as part of the needle thread necessary for forming the stitch. Accordingly, when the take-up lever returns to the uppermost position after the needle thread and the bobbin thread have been interlocked, an excessively high tension proportional to the length of the needle thread and used for forming the stitch among the length of the same stored by the loop taker is exerted on the needle thread; consequently, the bobbin thread is pulled out on the surface of the fabric being sewn deteriorating the quality of the stitch.